1. Field of the Invention
The present invention relates to an electromagnetic solenoid adapted to be coupled to a fluid control valve such as a flow-rate control valve, a fluid pressure control valve, a changeover valve or the like for controlling a flow rate, fluid pressure, or flow direction of fluid.
2. Description of the Prior Art
FIG. 4 is a cross sectional view showing a conventional electromagnetic solenoid. In this Figure, reference numeral 1 designates a cylindrical casing having a fixed iron core 2 integrally formed with its end wall 1a, the casing 1 being closed at its open end opposite the end wall 1a by an end plate 3. The casing 1, the fixed iron core 2 and the end plate 3 are adapted to jointly form a magnetic circuit. Housed in the casing 1 is a solenoid coil 4 which is wound around a winding frame or a bobbin 5 formed of synthetic resin. A movable iron core 6 is fixedly connected with an output rod 7 extending through and supported by the fixed iron core 2 for axial sliding movement relative thereto. Firmly fitted in the inner peripheral surface of the winding frame 5 is a sleeve 9 formed of a non-magnetic material for supporting the movable iron core 6 for axial sliding movement. The movable iron core 6 is biased toward the fixed iron core 2 under the action of a biasing spring 10.
The electromagnetic solenoid constructed in the above manner is to be connected with a fluid control valve which is generally designated by reference numeral 11. The fluid control valve 11 includes a valve housing 12 which has a stepped axial bore 13 formed therein. The stepped axial bore 13 includes a small-diameter portion 13a and a large-diameter portion 13b. The fixed iron core 2 of the electromagnetic solenoid is thread-engaged in the large-diameter bore 13b in the valve housing 12. The valve housing 12 has three fluid conduits 12a, 12b and 12c opening into the small-diameter bore 13a. Slidably fitted in the small-diameter bore 13a is a valve member 14 in the form of a spool for controlling the communication of the respective fluid conduits 12a, 12b and 12c with the small-diameter bore 13a. The valve member 14 is connected with the output rod 7 and urged in the leftward direction in FIG. 4 under the action of a return spring 15 which is stronger than the spring 10 so that the movable iron core 6 is biased in the direction away from the fixed iron core 2 when the solenoid coil 4 is not energized.
The conventional electromagnetic solenoid coupled with the fluid control valve 11 in the above manner operates as follows. When the solenoid coil 4 is energized, the movable iron core 6 is magnetically attracted toward the fixed iron core 2 against the biasing force of the return spring 15 arranged in the valve housing 12 so that the output rod 7 is likewise caused to move in the direction indicated by an arrow C, in FIG. 4 thereby controlling or changing the communication of the fluid conduits 12a, 12b and 12c with the axial bore 13. In this case, it is to be noted that the confronting surfaces of the fixed and movable iron cores 2 and 6 are tapered to form a conical configuration so that the magnetic attraction force generated by the solenoid coil 4 and acting between the fixed and movable iron cores 2 and 6 increases in proportion to both the amount of the advancing or leftward movement of the output rod 7 and an increase in intensity of the current flowing through the solenoid coil 4.
With the above-described conventional electromagnetic solenoid, however, there have been the following problems. Specifically, the solenoid coil 4 creates, upon energization thereof, a magnetic attraction force acting between the fixed and movable iron cores 2 and 6 so that the movable iron core 6 is displaced axially by an axial component of the magnetic force and radially by a radial component of the magnetic force which generally acts in a non-uniform pattern radially around the circumference of the movable iron core 6. As a result, the output rod 7 becomes more or less cocked and is thus subjected to a greater frictional resistance from the inner surface of the through bore 1b in the fixed iron core 2 so that the axial force required for causing a specified amount of axial movement of the output rod 7 during the advancing stroke (the rightward stroke in FIG. 4) thereof is made different from that during the return stroke (the leftward stroke in FIG. 4) in which the solenoid coil 4 is deenergized and the magnetic attraction force between the fixed and movable iron cores 2 and 6 disappears. This is clear in the graph illustrated in FIG. 5 in which the axial force of the output rod 7 is plotted as the ordinate and the amount of movement of the output rod 7 is plotted as the abscissa. In other words, the relationship between the axial force acting on the output rod 7 and the amount of resulting axial movement of the output rod 7 is such that hysteresis in the sliding motion of the output rod 7 relative to the axial force required during axial sliding reciprocation of the output rod 7 is great, that is the difference in the axial force required to cause a specified amount of axial movement of the output rod 7 during the advancing stroke and the returning stroke is great. Consequently, when the amount of axial movement of the output rod 7 is to be controlled in terms of the intensity of the current flowing through the solenoid coil 4, the amount of axial movement of the output rod 7 due to the coil current of the same intensity during the advancing stroke (the rightward movement in FIG. 4) of the output rod 7 is considerably different from that during the returning stroke (the leftward movement in FIG. 4) so that it is difficult to precisely control the amount of axial movement of the output rod 7 by adjustment of the coil current.
Moreover, powder of magnetic material, produced by the friction of repeated sliding reciprocations of the output rod 7 relative to the fixed iron core 2 is contained in the operating fluid and this powder is liable to be magnetically attracted and adhered to the fixed iron core 2, and particularly to the inner peripheral surface of the through bore 1b in the fixed iron core 2 and/or the outer peripheral surface of the output rod 7 so that smooth axial sliding movement of the output rod 7 relative to the fixed iron core 2 is considerably impaired by the thus adhered powder.
In addition, as the output rod 7 moves in the opposite axial directions, operating fluid in the large-diameter bore 13b in the valve housing 12 flows therefrom into a space 5a defined by the inner peripheral surface of the winding frame 5 through an annular clearance between the inner peripheral surface of the through bore 1b and the outer peripheral surface of the output rod 7 or vice versa. Consequently, the area of the fluid passage or annular clearance communicating between the large-diameter bore 13b in the valve housing 12 and the space 5a inside the winding frame 5 is limited, thus providing a relatively large resistance to the fluid flow passing through the annular clearance during axial movements of the movable iron core 6 with the result that smooth axial sliding motion of the movable iron core 6 is impaired reducing the responsiveness thereof to a material extent.